// @flow strict

import { SYMBOL_ASYNC_ITERATOR } from '../polyfills/symbols';

import inspect from '../jsutils/inspect';
import { addPath, pathToArray } from '../jsutils/Path';

import { GraphQLError } from '../error/GraphQLError';
import { locatedError } from '../error/locatedError';

import type { DocumentNode } from '../language/ast';

import type { ExecutionResult } from '../execution/execute';
import {
  assertValidExecutionArguments,
  buildExecutionContext,
  buildResolveInfo,
  collectFields,
  execute,
  getFieldDef,
  resolveFieldValueOrError,
} from '../execution/execute';

import type { GraphQLSchema } from '../type/schema';
import type { GraphQLFieldResolver } from '../type/definition';

import { getOperationRootType } from '../utilities/getOperationRootType';

import mapAsyncIterator from './mapAsyncIterator';

export type SubscriptionArgs = {|
  schema: GraphQLSchema,
  document: DocumentNode,
  rootValue?: mixed,
  contextValue?: mixed,
  variableValues?: ?{ +[variable: string]: mixed, ... },
  operationName?: ?string,
  fieldResolver?: ?GraphQLFieldResolver<any, any>,
  subscribeFieldResolver?: ?GraphQLFieldResolver<any, any>,
|};

/**
 * Implements the "Subscribe" algorithm described in the GraphQL specification.
 *
 * Returns a Promise which resolves to either an AsyncIterator (if successful)
 * or an ExecutionResult (error). The promise will be rejected if the schema or
 * other arguments to this function are invalid, or if the resolved event stream
 * is not an async iterable.
 *
 * If the client-provided arguments to this function do not result in a
 * compliant subscription, a GraphQL Response (ExecutionResult) with
 * descriptive errors and no data will be returned.
 *
 * If the source stream could not be created due to faulty subscription
 * resolver logic or underlying systems, the promise will resolve to a single
 * ExecutionResult containing `errors` and no `data`.
 *
 * If the operation succeeded, the promise resolves to an AsyncIterator, which
 * yields a stream of ExecutionResults representing the response stream.
 *
 * Accepts either an object with named arguments, or individual arguments.
 */
declare function subscribe(
  SubscriptionArgs,
  ..._: []
): Promise<AsyncIterator<ExecutionResult> | ExecutionResult>;
/* eslint-disable no-redeclare */
declare function subscribe(
  schema: GraphQLSchema,
  document: DocumentNode,
  rootValue?: mixed,
  contextValue?: mixed,
  variableValues?: ?{ +[variable: string]: mixed, ... },
  operationName?: ?string,
  fieldResolver?: ?GraphQLFieldResolver<any, any>,
  subscribeFieldResolver?: ?GraphQLFieldResolver<any, any>,
): Promise<AsyncIterator<ExecutionResult> | ExecutionResult>;
export function subscribe(
  argsOrSchema,
  document,
  rootValue,
  contextValue,
  variableValues,
  operationName,
  fieldResolver,
  subscribeFieldResolver,
) {
  /* eslint-enable no-redeclare */
  // Extract arguments from object args if provided.
  return arguments.length === 1
    ? subscribeImpl(argsOrSchema)
    : subscribeImpl({
        schema: argsOrSchema,
        document,
        rootValue,
        contextValue,
        variableValues,
        operationName,
        fieldResolver,
        subscribeFieldResolver,
      });
}

/**
 * This function checks if the error is a GraphQLError. If it is, report it as
 * an ExecutionResult, containing only errors and no data. Otherwise treat the
 * error as a system-class error and re-throw it.
 */
function reportGraphQLError(error) {
  if (error instanceof GraphQLError) {
    return { errors: [error] };
  }
  throw error;
}

function subscribeImpl(
  args: SubscriptionArgs,
): Promise<AsyncIterator<ExecutionResult> | ExecutionResult> {
  const {
    schema,
    document,
    rootValue,
    contextValue,
    variableValues,
    operationName,
    fieldResolver,
    subscribeFieldResolver,
  } = args;

  const sourcePromise = createSourceEventStream(
    schema,
    document,
    rootValue,
    contextValue,
    variableValues,
    operationName,
    subscribeFieldResolver,
  );

  // For each payload yielded from a subscription, map it over the normal
  // GraphQL `execute` function, with `payload` as the rootValue.
  // This implements the "MapSourceToResponseEvent" algorithm described in
  // the GraphQL specification. The `execute` function provides the
  // "ExecuteSubscriptionEvent" algorithm, as it is nearly identical to the
  // "ExecuteQuery" algorithm, for which `execute` is also used.
  const mapSourceToResponse = (payload) =>
    execute({
      schema,
      document,
      rootValue: payload,
      contextValue,
      variableValues,
      operationName,
      fieldResolver,
    });

  // Resolve the Source Stream, then map every source value to a
  // ExecutionResult value as described above.
  return sourcePromise.then((resultOrStream) =>
    // Note: Flow can't refine isAsyncIterable, so explicit casts are used.
    isAsyncIterable(resultOrStream)
      ? mapAsyncIterator(
          ((resultOrStream: any): AsyncIterable<mixed>),
          mapSourceToResponse,
          reportGraphQLError,
        )
      : ((resultOrStream: any): ExecutionResult),
  );
}

/**
 * Implements the "CreateSourceEventStream" algorithm described in the
 * GraphQL specification, resolving the subscription source event stream.
 *
 * Returns a Promise which resolves to either an AsyncIterable (if successful)
 * or an ExecutionResult (error). The promise will be rejected if the schema or
 * other arguments to this function are invalid, or if the resolved event stream
 * is not an async iterable.
 *
 * If the client-provided arguments to this function do not result in a
 * compliant subscription, a GraphQL Response (ExecutionResult) with
 * descriptive errors and no data will be returned.
 *
 * If the the source stream could not be created due to faulty subscription
 * resolver logic or underlying systems, the promise will resolve to a single
 * ExecutionResult containing `errors` and no `data`.
 *
 * If the operation succeeded, the promise resolves to the AsyncIterable for the
 * event stream returned by the resolver.
 *
 * A Source Event Stream represents a sequence of events, each of which triggers
 * a GraphQL execution for that event.
 *
 * This may be useful when hosting the stateful subscription service in a
 * different process or machine than the stateless GraphQL execution engine,
 * or otherwise separating these two steps. For more on this, see the
 * "Supporting Subscriptions at Scale" information in the GraphQL specification.
 */
export function createSourceEventStream(
  schema: GraphQLSchema,
  document: DocumentNode,
  rootValue?: mixed,
  contextValue?: mixed,
  variableValues?: ?{ +[variable: string]: mixed, ... },
  operationName?: ?string,
  fieldResolver?: ?GraphQLFieldResolver<any, any>,
): Promise<AsyncIterable<mixed> | ExecutionResult> {
  // If arguments are missing or incorrectly typed, this is an internal
  // developer mistake which should throw an early error.
  assertValidExecutionArguments(schema, document, variableValues);

  try {
    // If a valid context cannot be created due to incorrect arguments,
    // this will throw an error.
    const exeContext = buildExecutionContext(
      schema,
      document,
      rootValue,
      contextValue,
      variableValues,
      operationName,
      fieldResolver,
    );

    // Return early errors if execution context failed.
    if (Array.isArray(exeContext)) {
      return Promise.resolve({ errors: exeContext });
    }

    const type = getOperationRootType(schema, exeContext.operation);
    const fields = collectFields(
      exeContext,
      type,
      exeContext.operation.selectionSet,
      Object.create(null),
      Object.create(null),
    );
    const responseNames = Object.keys(fields);
    const responseName = responseNames[0];
    const fieldNodes = fields[responseName];
    const fieldNode = fieldNodes[0];
    const fieldName = fieldNode.name.value;
    const fieldDef = getFieldDef(schema, type, fieldName);

    if (!fieldDef) {
      throw new GraphQLError(
        `The subscription field "${fieldName}" is not defined.`,
        fieldNodes,
      );
    }

    // Call the `subscribe()` resolver or the default resolver to produce an
    // AsyncIterable yielding raw payloads.
    const resolveFn = fieldDef.subscribe ?? exeContext.fieldResolver;

    const path = addPath(undefined, responseName, type.name);

    const info = buildResolveInfo(exeContext, fieldDef, fieldNodes, type, path);

    // resolveFieldValueOrError implements the "ResolveFieldEventStream"
    // algorithm from GraphQL specification. It differs from
    // "ResolveFieldValue" due to providing a different `resolveFn`.
    const result = resolveFieldValueOrError(
      exeContext,
      fieldDef,
      fieldNodes,
      resolveFn,
      rootValue,
      info,
    );

    // Coerce to Promise for easier error handling and consistent return type.
    return Promise.resolve(result).then((eventStream) => {
      // If eventStream is an Error, rethrow a located error.
      if (eventStream instanceof Error) {
        return {
          errors: [locatedError(eventStream, fieldNodes, pathToArray(path))],
        };
      }

      // Assert field returned an event stream, otherwise yield an error.
      if (isAsyncIterable(eventStream)) {
        // Note: isAsyncIterable above ensures this will be correct.
        return ((eventStream: any): AsyncIterable<mixed>);
      }

      throw new Error(
        'Subscription field must return Async Iterable. ' +
          `Received: ${inspect(eventStream)}.`,
      );
    });
  } catch (error) {
    // As with reportGraphQLError above, if the error is a GraphQLError, report
    // it as an ExecutionResult; otherwise treat it as a system-class error and
    // re-throw it.
    return error instanceof GraphQLError
      ? Promise.resolve({ errors: [error] })
      : Promise.reject(error);
  }
}

/**
 * Returns true if the provided object implements the AsyncIterator protocol via
 * either implementing a `Symbol.asyncIterator` or `"@@asyncIterator"` method.
 */
function isAsyncIterable(maybeAsyncIterable: mixed): boolean {
  if (maybeAsyncIterable == null || typeof maybeAsyncIterable !== 'object') {
    return false;
  }

  return typeof maybeAsyncIterable[SYMBOL_ASYNC_ITERATOR] === 'function';
}
